When shooting 2D objects (e.g., artwork), what is the easiest way to make sure that the optical axis of the camera is aligned perpendicular to the object? That is, how do I avoid any perspective distortion?

Of course I could just try to stare at the tiny viewfinder or the LCD screen and try to guess when it is aligned properly, but all too often you notice the tiny distortions only afterwards when you look at the photos on a large screen (for example, if the object is a perfect square and you crop it tightly, tiny distortions result in uneven margins around the object). Tethered shooting is one option, but what other tools or tricks one could use?

The best solution so far is the following: Replace the object with a mirror. Then (using liveview & zoom), align the camera so that I can see the reflection of the very centre of my lens precisely in the middle of the picture. Then put back the object and take the picture.

It works but it is not perfect; for one, it is not that easy to both align the camera perpendicular to the object and simultaneously keep the horizon level. The horizon is of course easy to fix in post, but it would not hurt to get it right directly.

Jukka - there is one keypoint missing in your question - what size of artwork are you talking about. Shooting a postcard size art can be easily done with a tripod facing down and the piece on the floor. However, a wall sized painting is much harder to shoot this way (this is what I have in my mind in my answer).
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ysapAug 12 '11 at 17:14

@ysap: I would not like to restrict the question too much; I will be happy to hear any tricks and tips, even if some of them are only applicable in specific situations (such as small artwork).
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Jukka SuomelaAug 12 '11 at 22:13

3 Answers
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Create a level surface to lay the artwork flat on - you can use a spirit level to make sure it is level.

If the artwork itself isn't very flat, either get a clean piece of glass larger than the artwork to lay on it or mount it to something flat. If using the glass approach you need to light the artwork with lamps to the left and right of the subject facing it at 45 degrees.

Then use a tripod with a built in spirit level (or get one of the hotshoe mounted levels) and level that looking directly down at the artwork.

This would more or less reproduce the environment I used to shoot artworks for greetings cards on back in my lab days except we had an industrial vertical camera mount with flat surface for the artwork attached below it, and I was using a bellows 5x4 plate camera :)

A tripod with level is a possible solution, but if one is not available, then using a camera mounted level is a better solution.

In some cameras (Canon 7D for example) there is a built in electronic level that can be used as well.

UPDATE: following a couple of comments by @Itai and @MikeW in @Maynard Case's answer, lets put some things straight (pun intended):

You will never get perfect alignment consistently. "Perfect" here is relative, and I assume some error is allowed. This is not a NASA lab we are dealing with here. Also assuming the work itself is hanged pretty perfectly vertical.

You will always have geometry distortions. No matter what. The center of the artwork is closer to the camera than the midpoints of the edges, which themselves are closer than the corners, so even if everything is perfectly symmetrical, you will have some distortion. Heck, your lens will induce some barrel/pincushion distortion as well. The best thing that you can do is to minimize this effect by using as long a focal length as possible.

The mirror trick is completely unnecessary and redundant. If you are able to replace it with a mirror, then you certainly are able to just measure the center of the artwork (say, crosspoint of the two diagonals). Then, with a leveled camera (with whatever means already mentioned), use 100% live view to align the centerpoint to the center of the zoomed-in frame. The error margins will be at least as good as with the mirror trick.

UPDATE 2 (for OP's request): If one wants to align the camera in the left-right plane as well as up-down plane, then it is possible to use two strings of identical length attached to the artwork's corners. form a triangle where the it head is tied to the tripod's mount screw. Then, together with the center point that was found earlier and the camera being level, you have a left-right alignment as well.

You can also attach four strings and form a pyramid. Then, position the camera such that the center of your lens' front element is at the head of the pyramid. While the camera is pointing at the center of the artwork, you can't have better alignment than this!

@ysap: I think you have misunderstood the mirror trick: It is not related to centering the artwork; centering is easy with live view, and fine-tuning is easy with cropping. However, even with the artwork centered and the camera perfectly level, the angle between the optical axis of the camera and the artwork might be, for example, 85° instead of 90°, and this is where the mirror trick certainly helps. (By the way, having the optical axis properly aligned not only prevents perspective distortions, but also makes sure that the focal plane is aligned with the artwork – no blurry corners.)
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Jukka SuomelaAug 12 '11 at 9:12

@Jukka - Sorry, but I think you have misunderstood my answer. If the artwork is hanged on a standard wall, then gravity makes sure that it is vertical (surely better than 5 degrees). Leveling the camera makes sure it is perpendicular to the artwork then. The mirror trick won't get you better than this. And surely hanging a mirror on or instead the artwork won't give you an exactly parallel surface too the artwork - not more than under the above assumptions.
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ysapAug 12 '11 at 12:59

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@Jukka - you are right, but this is easily solvable as well within the acceptable tolerances. Attach two same length strings to the corners of the artwork and form a triangle where its head is at the camera's position (the tripod's mounting screw for that matter). This will locate the camera exactly in front of the midpoint of the artwork that you found previously.
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ysapAug 12 '11 at 14:20

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@ysap: Now that is a good example of the kind of tricks that I am looking for! Perhaps you could post this as (another) answer?
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Jukka SuomelaAug 12 '11 at 14:36

This makes no sense at all. None of the level matter until the object is perpendicularly on the ground!
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ItaiAug 12 '11 at 0:46

@Itai - as is usually the case with artwork! Gravity makes sure it is as perfectly aligned as you can get, or at least as aligned as the wall's compliance with the construction code. Note the keypoint in Maynard's answer - "assuming the artwork is hanging on a vertical wall". I think the -1 is not justified here.
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ysapAug 12 '11 at 0:58

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Levels would make sure the camera is level, but doesn't assure it's at the right height, or the right position left and right. You still need some way (see the OP mirror trick) to make sure the camera is centered on the centre of the artwork.
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MikeWAug 12 '11 at 4:48

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Of course one possibility would be to place the artwork flat on the floor and then use 2 levels to make sure that the camera is pointing straight downwards. This has the same restriction as the mirror trick, though: the artwork might be rotated in the picture. That is, you get 2 axes of rotation right but 1 axis might be still wrong.
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Jukka SuomelaAug 12 '11 at 5:33

Another possibility would be to shoot artwork that is hanging on a wall, and combine the mirror trick (to make sure that the optical axis of the camera is perpendicular to the wall) and a spirit level (to make sure that the horizon is level). But this gets a bit fiddly and I was hoping that there might be a more straightforward approach.
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Jukka SuomelaAug 12 '11 at 5:37